1. Field of the Invention
The present invention relates to a positive electrode active material and to a nonaqueous electrolyte secondary battery equipped with a positive electrode containing the positive electrode active material.
2. Description of the Related Art
In recent years, a portable information terminal has been developed and being propagated rapidly. With propagation of the portable information terminal, a research and development of a nonaqueous electrolyte secondary battery used as a power source of the portable information terminal is being carried out vigorously so as to put the secondary battery to the practical use. Known is a lithium ion secondary battery, which is an example of the nonaqueous electrolyte secondary battery, comprising a positive electrode, a negative electrode, a separator arranged between the positive electrode and the negative electrode, and a liquid nonaqueous electrolyte impregnated in the separator. The lithium ion secondary battery, which has been put to the practical use, has a discharge voltage of about 4V.
On the other hand, the development of a nonaqueous electrolyte secondary battery exhibiting a discharge voltage higher than 4V has already been started. A nonaqueous electrolyte secondary battery comprising LiCoPO4 or LiNiPO4 as the positive electrode active material is known to exhibit a high discharge voltage of about 5V.
The theoretical discharge capacity, which is obtained when a lithium ion is inserted into and extracted from the active material, is about 170 mAh/g in each of LiCoPO4 and LiNiPO4. However, the discharge capacity that is actually obtained is about half the theoretical discharge capacity noted above. In addition, each of LiCoPO4 and LiNiPO4 is defective in that the diffusion rate of the lithium ions within the crystal is low, with the result that, if the charge-discharge is carried out with a high current density, it is impossible to obtain a large discharge capacity.